Effects of probe sampling rates on sample composition

Abstract The change of sample composition that occurs during non-iso-kinetic sampling in a flow with a concentration gradient was studied theoretically, and the results were confirmed by experiment. Insertion of the probe into the stream and the change of the concentration gradient in the region outside of the probe due to flow distortion are the factors that cause the change of the sample composition. They have opposing effects, and the sample composition is determined accordingly. Since the profile of the concentration distribution measured by non-iso-kinetic sampling is distorted from the unperturbed one, an incorrect value for the concentration gradient may be evaluated from it, while the correct gradient at a definite distance upstream is evaluated from the distribution measured by iso-kinetic sampling. Sampling by a large probe in a flow with a concentration gradient may produce large error. The relative error decreases with an increase in the sampling rate within a finite range and decreases with decreasing probe width.